The field is internal combustion engines, and is particularly related to constructions for thermal management of pistons. In some aspects, the field includes internal combustion engines in which the end surface of a piston crown is insulated from the interior of the piston by cavities embedded in the crown.
With reference to FIG. 1, an internal combustion engine is illustrated by way of an opposed-piston engine that includes at least one cylinder 10 with a bore 12 and longitudinally-displaced exhaust and intake ports 14 and 16 machined or formed therein. Fuel injector nozzles 17 are located in or adjacent injector ports that open through the side of the cylinder, at or near the longitudinal center of the cylinder. Two pistons 20, 22 are disposed in the bore 12 with their end surfaces 20e, 22e in opposition to each other.
Operation of an opposed-piston engine with one or more cylinders such as the cylinder 10 is well understood. In this regard, and with reference to FIG. 2, in response to combustion occurring between the end surfaces 20e, 22e the opposed pistons move away from respective top dead center (TDC) positions where they are at their closest positions relative to one another in the cylinder. While moving from TDC, the pistons keep their associated ports closed until they approach respective bottom dead center (BDC) positions in which they are furthest apart from each other. The pistons may move in phase so that the exhaust and intake ports 14, 16 open and close in unison. Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times.
As the pistons continue moving away from each other, the intake port 16 opens while the exhaust port 14 is open and a charge of pressurized air (“charge air”) is forced into the cylinder 10, driving exhaust gasses out of the exhaust port 14. The displacement of exhaust gas from the cylinder through the exhaust port while admitting charge air through the intake port is referred to as “scavenging”. Because the charge air entering the cylinder flows in the same direction as the outflow of exhaust gas (toward the exhaust port), the scavenging process is referred to as “uniflow scavenging”.
As the pistons move through their BDC locations and reverse direction, the intake and exhaust ports are closed by the pistons, scavenging ceases, and the charge air in the cylinder is compressed between the end surfaces 20e and 22e. Typically, the charge air is swirled as it passes through the intake port 16 to promote scavenging while the ports are open and, after the ports close, to mix the air with the injected fuel. Typically, the fuel is diesel which is injected into the cylinder by high pressure injectors. With reference to FIG. 1 as an example, the swirling air (or simply, “swirl”) 30 has a generally helical motion that forms a vortex in the bore which circulates around the longitudinal axis of the cylinder. As best seen in FIG. 2, as the pistons advance toward their respective TDC locations in the cylinder bore, fuel 40 is injected through the nozzles 17 directly into the swirling charge air 30 in the bore 12, between the end surfaces 20e, 22e of the pistons. The swirling mixture of charge air and fuel is compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e when the pistons 20 and 22 are near their respective TDC locations. When the mixture reaches an ignition temperature, the fuel ignites in the combustion chamber, driving the pistons apart toward their respective BDC locations
In some aspects of internal combustion engine construction it is desirable to utilize pistons with highly contoured end surfaces that interact with swirl and with squish flow from the periphery of the combustion chamber to produce complex, turbulent charge air motion that encourages uniform mixing of air and fuel. However, combustion imposes a heavy thermal load on these pistons. The highly contoured end surfaces create non-uniform thermal profiles that are not suitably cooled by conventional forced cooling configurations, leading to asymmetrical thermal stress, wear, and piston fracture.
Constructions for cooling a piston with a highly contoured end surface can include one or more internal galleries that conduct liquid coolant along the rear surface of a piston crown. Desirably, these constructions tailor internal cooling of the piston so as to deliver a relatively high cooling capacity to the hottest portions of the crown as compared with the cooler parts. For example, one such construction combines impingement cooling of the hottest portions of the interior with shaped gallery spaces that conduct flows of coolant to other internal portions of the piston. However, there is a price to be paid. In this regard, cooling the piston interior increases the flow of heat from the piston end surface to the piston interior, thereby reducing the amount of heat that is available to convert into useful work. Accordingly, while it is desirable to cool the interior of the piston in the vicinity of the piston end surface, it is also desirable to limit the amount of heat transferred from the end surface into the piston's interior. The trade-offs in piston construction with respect to cooling the interior of the piston while reducing loss of heat through the end surface form the basis of piston thermal management.